Transcript Lessons Learned Building the BeagleBoard

Hardware Lessons Learned from
Building Beagle
Gerald Coley
ESC-401
Topics Covered
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What’s in a name?
Specification
Schematic
PCB Layout
Assembly
Testing
Support
Implementing Lessons Learned
Questions and discussion
The Name:
Where did we get the name
BeagleBoard?
How did we arrive at BeagleBoard?
• Every project needs a name
• Named for my dog.
– Just “temporary”
– We will fix it later
– Well, It Stuck
• A Beagle is:
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Like Tux, non threatening
Curious
Loyal
Fun
And gives back as much as it receives
• Lesson Learned…If you name it, it will stick!
So, we need an acronym
• Bring your own peripherals
• Entry-level
• ARM Cortex-A8
• Graphics & DSP
• Linux and open source
• Environment for SW innovators
The Specification:
A buffet
or
just meat and potatoes?
The Goals
• Get OMAP3530 into people’s hands
• Low Cost <$150
– Under the spousal radar
– Reach as many people as we can
• Small Size
– Keeps the cost down
– “Cool” Factor
• Internally focused
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Cortex-A8 + NEON
DSP
3D Graphics
Accelerators
First Pass Features
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OMAP3530 Processor
3.5” LCD
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S-Video
Keypad
2 Serial Ports
SD/MMC
Ethernet
Camera
128MB DDR
128MB NAND
USB
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VGA/QVGA
Touch screen
Host
OTG
USB/DC Power
Stereo In/Out
Battery w/Charger
Full Expansion Bus
COST: $1500
Size:
5” x 8”
Something has to go!!
Bring Your Own
• Not everyone needs everything
• Everything is needed
• Bring your Own
– User adds only what they need
– They don’t have to pay for what they already
have
– Allows for maximum exposure to HW
• More than just one component supplier
Final Pass Features
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OMAP3530
3.5” LCD.-VGA/QVGA
– Touch screen USB Touch Screen 
DVI-D VGA, SVGA, XGA 
S-Video 
Keypad
– 1 button USB Keyboard & Mouse 
2 Serial Ports
– 1 Port USB & Expansion 
SD/MMC 6 in 1 Slot 
Ethernet
– USB Dongle & WiFI 
Camera
– USB Camera 
128MB DDR 
128MB NAND 
USB Host OTG
USB/DC Power 
Stereo In/Out 
Battery w/Charger
– USB Battery Adapter 
Size 5” x 8” 3” x 3”  
Full Expansion Bus
– Standard buses 
Standard on Beagle
Bring your own
COST: $149
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Just meat and potatoes,
but still room for dessert!!
The Schematic
Play It Safe
• Leverage everything we can
– Used OMAP35xx EVM as base
• Maintain compatibility with other platforms
– Common hookup
– Subset used the same
• Keep the component count low
• Use what is proven to work
• Leave off what is not needed
Leveraged Advantages
• OMAP35xx Processor Symbol/Design verified
• TWL4030/TPS65950 Symbol/Design verified
– Removed a lot of features here
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SD/MMC Verified
DVI-D verified
UART Verified
Lowered the overall risk
PCB Layout
Rules We Used
PCB
PCB
Design
FAB
TI
Memory
Vendors
PCB
ASSEMBLY
Failure to include the fab and assembly team members can prove costly since choices
made early in the design will adversely impact the final cost of the assembled board
The Challenges
• OMAP35xx Package
– .4mm Pitch
– Routing
• TPS65950(TWL4030)
– .4mm pitch PMIC
• POP Implications
• Limited area for connectors
– Location driven and not layout driven
OMAP3530 Package
What Exactly Is POP?
Memory
OMAP
PMIC TPS65950 (TWL4030)
Fixed Location of Connectors
• Routing complicated by locations
– Connector not always on the best side
– We tried to make this as easy as possible
• Not the best locations for routing
– The DVI-D was the longest route
• We need to keep the layer count down
– Our goal was to stick with 6 layers
Recommended Trace and Via Connections
BGA
via
Trace should never
be larger than pad
Don’t gang BGA
pads with ground
plane
Poor
Good
Better
Better
Watch for solder traps
Best
Blind and Buried Vias
Definitions and Terminology
• Through hole via has access to both external layers
• Buried via provides connection within inner layers
• Blind via does not pass through the entire board
Via-in-Pad
Places the via directly in the BGA pad
Greatly improves board routing
Has special requirements for manufacturability
Given a 10mil BGA pad
• Use a 4mil microvia
• Laser drilled
• Via must be filled or capped to minimize void formation
Via-fill Material
• Check with board fabricator for their preferences
• Fill material can be conductive or non-conductive
• Via fill material’s particle size must be <1mil for complete penetration
You must validate your board fabricator’s capability to reliably build with this
class of technology
.4mm Pitch
• No routing between pads
• Use top layer routing on outside pads
• Use vias-in-pad wherever needed
Soldermask
• Exposed pad should be the same size as
the pad on the mounted device
• Need to make the pad larger to add
stability
• Calls for soldermask defined pads
Surface Finish
The PCB surface finish provides a coating over the outer layer copper
that prevents oxidation and provides an electrically conductive
surface.
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Organic Solderability Preservative (OSP) Thin layer of organic material to
prevent copper oxidation. It is removed by the assembly flux. Does not
tolerate multiple heat cycles.
Immersion Tin (ImSn) Thin layer of tin directly on top of the copper
surface. Produces an extremely flat surface for mounting of surface mount
components. Downside is the possible formation of Tin whiskers.
Immersion Silver (ImAg) Thin layer of silver directly on top of the copper
surface. Produces a very flat surface. Compatible with no-clean assembly
processes. Maintains high solderability after multiple heat cycles. Downside
is that the plating will tarnish over time.
Electroless Nickel Immersion Gold (ENIG) Commonly used finish that is
nice and flat for fine-pitch devices. Not easily reworked and more
expensive.
Hot-air solder leveling (HASL) Immersing the PCB in solder. It is
inexpensive and widely available, but it is not flat and therefore does not
work well with fine-pitch devices
Final BeagleBoard Design
TFP410
OMAP3 Triton-2
OMAP3530 BGA Footprint
Pad Type
Solder Mask Defined
Pad Pitch
A
400um
Pad Opening
B
254um (10mils)
Pad Dia
C
280um (11mils)
Mask Web
D
150um
Via-In-Pad Topology
• Stacked Microvia-In-Pad
• Pad 11mils copper (10mil SMD)
BeagleBoard does not use buried vias
• Top-Layer2 stacked with Layer2-Layer-3
• Top-Layer2
• Laser Drill 6mil (0.152mm)
• Solder-mask defined pads
• Non-conductive epoxy via plugs, plated over and planarized
OMAP3
Blind/Stacked
1-3
Blind
1-2
Top Soldermask
Top Signal
Lyr2-GND
Lyr3-SIG
Lyr4-SIG
Lyr5-PWR
Bottom Signal
Bottom Soldermask
Standard Vias
• Thru-hole via from top to bottom
• 18mil radius pad
• 8 mil drilled hole
• Plugged, plated and planarized
•Must be level
•No dimples
Top
Lyr2-GND
Lyr3-SIG
Lyr4-SIG
Lyr5-PWR
Not To Scale
Bottom
Top Layer-Signal
(Area under OMAP3 Chip)
Via 1-6
V-I-P
BGA
PAD
3mil trace
10mil trace
Bottom Layer
Bypass Capacitor Locations
Capacitor Spec
Type:
Ceramic X7R
Size:
402
Value:
0.1uF
Via 1-6
Component
Pad
•Min trace width 3 mils
Bypass Capacitor
Location and
Solder Pads
PCB Fabrication Concerns
• High Board cost
– Because they can charge more
– Unknown = $$$$$
– Does not necessarily mean the production $$
will be high
• Unfamiliar with fine pitch
• Soldermask registration critical
Board Assembly:
Putting it all together
POP Assembly Concerns
• POP was the number one concern
• Nitrogen and air were the two options
• What should we do?
– Picked the one that was the most applicable
to as many assembly houses as
possible….Air
• POP soldering in air
– Air #1
– Air #2
Mounting .4mm Parts
• No real concerns
• Equipment can handle it
• Warping of the processor due to POP was
a possibility
– Saw no reason to be concerned
• There could be an issue with shorts if
soldermask not correct
• We were comfortable
Solderpaste Equipment
Pick and Place Machine
My DATA (MY9)
D-014-1486 F30
Reflow Oven
Heller EXL
X-Ray Machine
Glenbrook Technologies
RTX-113
POP Mounting Configurations
• Method A
– POP onto OMAP3 first reflow
– Back side Second reflow
– Top side third w/POP+OMAP3530 reflow
• Method B
– Mount back side  reflow
– Mount top side with OMAP3530  reflow
– Mount Memory w/ Pick & Place  reflow
• Method C
– Mount backside  reflow
– Mount topside with OMAP3530 and POP  reflow
Method A Process
Hand
Mount
Top
GEL
POP
OMAP3
Mount w/SMT
OMAP3
Paste
SMT
Reflow
Bottom side of board
Hand
Assembly
Paste
Reflow
Top side of board
Method B Process
Paste
SMT
Paste
Reflow
Top side of board
Bottom side of board
Apply
Gel
SMT
POP Memory
SMT
Reflow
Hand
Assembly
Reflow
Method C Process
POP Memory
Paste
SMT
Reflow
Dipping Arm is
used to apply
paste
POP Memory
Paste
SMT
Top side of board
Bottom side of board
Hand
Assembly
Reflow
POP Mounting Analysis
• Method A
– No real issues seen
– We abandoned it when we ran into issues
– Issues were not related to this process
• Method B
– Final method adopted
– Have had excellent results
• Method C
– Dipping Arm…POP Memory dipped into GEL
– Have a lot of high volume customers doing this
– Did not try as we had no dipping arm
Panelization
• Method A
– 4 boards per panel
– Used it on the first runs
– Had concerns about warping across the
scoring that could cause solder shorts
– Nothing ever proven that this was an issue
• Method B
– One board per panel
– Method chosen
Other Parameters
• Stencil thickness
– .4mils
• Solder Paste
– AMTECH LF -4300 Lead Free
– Chemistry Sn96.5/Ag3.0/Cu.5
• Tacky paste flux (memory)
– Amtech Tacky Solder Flux
Reflow Profile Diagram
Temperature - Degrees C
Peak
250
200
150
100
50
Preheating
stage
Time
Reflow
Cooldown
Board Profile Settings
BOT
TOP
POP
# Zones In Oven
18
18
18
Pre-Heat Temp.
145 OC
170 OC
150 OC
Pre-Heat Dwell
98 sec
95 sec
100 sec
Reflow
118 sec
115 sec
120 sec
Duration Peak
18 sec
15 sec
20 sec
Peak Reflow Temp.
245 OC
260 OC
250 OC
8 OC/sec
8 OC/sec
8 OC/sec
52cm/s
55cm/s
50cm/s
Cool Down
Speed
Issues on 1st Run
• Solder Shorts under the processor
– Always a power rail
• 10% Yields
• Could it be related to….?
– POP Package
– Profile
– PCB
– Parts
– Solder
Solder Shorts
2nd Run
• What is the easiest thing to change?
– Adjusted the temperature profile
• Same results
• 90% Fail
• Shorts under processor
3rd Run
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Shotgun approach
Changed PCB Vendor
Went to single board penalization
Changed POP method from A to B
Kept the original profile
Results
– 96% Yields
– No Shorts
Analysis of 2nd Run
• What was the issue?
• Gut said PCB soldermask
– Contacted first supplier
– No issue w/soldermask per supplier
• Ordered more board and ran again with all other
methods from Run 3
– Same problem; 90% fail
• Ordered shotguns and raided first supplier
– They admitted that they opened the soldermask
– Had them run another batch w/o changing soldermask
• Result
– 96% yields
– No shorts
Good/Bad Soldermask
GOOD
BAD
Other Issues Along the Way
• SMT PCB audio connectors can rip off
– Only happened on two boards
– Pads detached from the board
– Added clear epoxy to connectors
– Adding vias to pads to add strength (Rev B6)
• SMT USB connectors can rip off
– Only happened on one board
– Adding vias to pads to add strength (Rev B6)
Final Analysis
• Soldermask is the key issue
– PCB suppliers do their own thing
– Make sure that the thing they do is your thing!
• All three POP methods should work
• Panalization should not be an issue, but not
confirmed
– It will be an issue if the soldermask is bad
• Beagle profile is a good place to start
– Will need to be adjusted based on equipment used
Board Testing Process
Inspection Points
• PCB
– Visual inspection
• Solder Paste
– Bottom side application
– Top side application
• Shorts
– After re-flow
• AOI
– After final assembly
• Manual inspection
– Final inspection
Assembly Inspection Points
PCB
Paste
Paste
SMT
Paste
Reflow
SMT
Reflow
Top side of board
Bottom side of board
Apply
Gel
Paste
POP Memory
SMT
Reflow
Shorts
AOI
Hand
Assembly
Visual
Shorts
PCB Check
• Check soldermask
– Overlap (Soldermask defined pads)
– Soldermask over vias
• Check finish
– Discoloring of finish
• Dimples in via in pads
– Must be smooth
• Check Pads
– Look for dimples in the vias in pad areas
Solderpaste Inspection
Check paste for smooth application
Check for missing paste (not sticking to PCB)
Shorts Test
• Implemented when we had solder shorts
• Easy check for main issues we have seen
• Check across the caps using a ohmmeter
– Run as a spot check
– Can be done at 100%
AOI Machine
Testronics 505 Machine Vision
-Works off of a known good board
-Check for part orientation
-Check for missing parts
Final Inspection
•Bad soldering
•Contamination
•Misaligned Parts
•Wrong parts
Functional Test
•Tests all interfaces on
the board
•Program NAND with
XLoader and UBoot
•Tested pre and post
burn-in
Burn-In
•72 Hour Burn In
•Room temperature
•Running UBoot
Final Results
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96%+ yield
Kept single board penalization
Kept assembly POP process
Kept Profile
Now have three suppliers of PCB
– DDI (Dynamic Details, Inc)
– MEI
– Streamline
REV A...Each Revision
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Only 50 units built
Rev A...Initial Version
Rev A1…Normalized LED brightness levels
Rev A2...Changed resistor loading options for S-Video
Rev A3…Lowered pull-up values for the I2C busses
Rev A4...Lowered value of USB cap due to turn on issues
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Too much current drain
• Rev A5...Incorrect inductor value on TPS65950 switchers
– Typo in BOM
• Issues to be fixed:
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DC Voltage jack
1K pull-up on wait line
Plated through hole issues
Remove 4 test points
User0 and User1 LEDS shorted
REV B ...Each Revision
• Rev B1…Initial Release
– Fixed outstanding Rev A issues
• Rev B2…USB Host not working reliably
– Removed from BOM and assembly
– Questions around the layout
• Rev B3…Added a few caps back in from B2
• Rev B4...Some USB HUBS not connecting on OTG Port
– Noise level too high…Added a capacitor onto USB power rail
• Rev B5...Serial Port disconnects after a while
– Removed a capacitor on the 32KHZ Clock
• Rev B6…PCB spin to change package of U9 and U11
• Issues To be Fixed:
– USB Host
Big Issue Emerges REV B
• Failures of the serial port and EDID
• Traced to a bad package on TXS0102
– Delamination issue
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60% Failure rate
Started 72 burn in process to weed out
Still have 1% making it out
Only solution is to move to a different
package
1st Rev C Proto
• Attempted to fix USB Host
– Significant improvement
– Still not 100%
• This version became the board for Rev B6
– Replaced U9 and U11 package
– Changed Revision to B1
2nd Rev C
• PCB In FAB
• Moved USB Host to port 2
– Aligned with other platforms
– Success on Port2
• Added native LCD access
– Lot of demand for this feature
– Ability to interface to different displays with
small paddle boards (LVDS, TTL, etc.)
– Kept current expansion header
Support Process
Key Issues from Users
• Serial Port Issues
– Confusion over cables
– Serial port failures
• OTG to host connectors
– Need “special” cable
– Brought on by lack of USB Host port
• Connecting another video output
– LCD
The Returns
RMA By Subsystem
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Audio…………..0
DVI-D…………..2
Serial Port……..19
Memory….……..0
S-Video….……..0
SD/MMC……….0
USB OTG……...0
User Issue……..9
PMIC……………3
Nothing Found...1
34 Total……..1.7%
Serial Port…..1.0%
The Serial Ports
• 60% of returns for this reason
• Issue with a small number of boards due
to part problem
• This issue masks other real issues
– Wrong IDC Cable
– Wrong SERIAL Cable
– Wrong terminal setup
– Well, it works on that other board?
Implementing the Lessons
Learned:
What’s Next?
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Changes Being Made
• Documentation
– Troubleshooting Section for Serial Port
– Pictures
– Step by Step process
• Diagnostic SW
• New Translator Package
– PCB Change REV B6
• LCD Interface (Rev C)
Design Material Available
http://beagleboard.org/hardware/design
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Schematics
– PDF Version
– OrCAD Version
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http://www.beagleboard.org/uploads/BEAGLE_ORCAD_B4.zip
BOM
– Excel
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http://www.beagleboard.org/uploads/BBSRM_B5.pdf
http://www.beagleboard.org/uploads/Beagle_BOM_B4.xls
PCB Files
– PCB Gerber Files http://www.beagleboard.org/uploads/Beagle_Allegro_B.zip
– PCB Allegro Files http://www.beagleboard.org/uploads/Beagle_Gerbers_B.zip
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Viewers
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PDF
OrCAD
Gerber
Allegro
PCB Design Guideline http://focus.ti.com/dsp/docs/dspsupporttechdocsc.tsp?sectionId=3&tabId=409&familyId=1526&abstractName=spraav1
System Reference Manual http://www.beagleboard.org/uploads/BBSRM_B5.pdf
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Rev B6 on its way
Can you do OMAP3?
• Take the BeagleBoard and build it
– Verifies if the PCB Fabricator can do it
– Verifies if the Assembly house can build it
– All material needed is provided
• All needed SW is available
– Diagnostics
– Kernels
– Distributions
Thanks To:
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Contract Manufacturer
CircuitCo www.circuitco.com
PCB Fabricators
Marcel Electronics mei4pcbs.com
Dynamic Details www.ddiglobal.com
Streamline www.streamlinecircuits.com
PCB Layout
ION Design www.iondesign.com
Keith Gutierrez , Texas Instruments
Questions and Discussion